1. Field of the Invention
The present invention relates to a clutch operation assisting device for reducing the operating force for operating a friction clutch when disengaging the transmission of torque by the friction clutch, and a power unit and a straddle-type vehicle that are equipped with the clutch operation assisting device.
2. Description of Related Art
A power unit of a motorcycle may be equipped with a friction clutch for connecting/disconnecting the transmission of torque from an engine to a transmission. The friction clutch typically includes a clutch shaft, and a friction plate and a clutch plate that overlap each other in the axial direction of the clutch shaft (hereinafter, referred to as the clutch shaft direction). The friction clutch also includes a pressure plate which brings the friction plate and the clutch plate into pressure contact with each other, and a clutch spring for urging the pressure plate. The clutch spring keeps the friction clutch in a clutch-in state, enabling the transmission of torque.
Further, the friction clutch has a clutch release mechanism. The clutch release mechanism serves to release the pressure of the pressure plate by the clutch spring, and is connected to a clutch lever via a clutch wire. When the rider grips the clutch lever, the pressure plate slides against the urging force of the clutch spring, thus disengaging transmission of torque from the friction plate to the clutch plate.
In the case of a friction clutch used in a high output/high RPM engine, in order to achieve increased torque capacity, it is desirable to set the mounting load of the clutch spring high. However, since the clutch lever is operated with a human hand, increased mounting load of the clutch spring increases the burden on the rider when operating the clutch lever.
To alleviate this problem, in the related art, for example, there is known a clutch operation assisting device in which an assist mechanism for reducing the operating force for operating the clutch lever is attached to a clutch release mechanism to which a clutch wire is connected (for example, see JP-A-Hei 7-132872 and JP-A-Sho 55-94028 below).
The clutch operation assisting device disclosed in JP-A-Hei 7-132872 is arranged outside of a power unit. On the other hand, there is also a demand for such a clutch operation assisting device to be arranged inside the power unit. JP-A-Sho 55-94028 discloses a construction in which the clutch operation assisting device is arranged inside the power unit.
FIG. 13 depicts the clutch operation assisting device disclosed in JP-A-Sho 55-94028. It includes a rotatable push lever 525 connected to a clutch wire 528, and an auxiliary spring 529 for reducing the operating force for operating a clutch lever. One end 529a of auxiliary spring 529 is connected to push lever 525, and the other end of auxiliary spring 529 is connected to a connecting member 531 that is formed in a semi-arcuate shape. Push lever 525 is fixed to a push screw 517 positioned on a straight line extending through the plane of FIG. 13, and rotates integrally with push screw 517. Connecting member 531 is rotatably supported on and rotates around a pin 530.
When the clutch lever is in a so-called freeplay condition (a condition where no load is exerted even when the clutch lever is pulled), as indicated by the solid line in FIG. 13, auxiliary spring 529 is positioned on a straight line M1 extending between the rotation center C0 of push lever 525 and pin 530. Thus, the expansion/contraction direction of auxiliary spring 529 passes through rotation center C0 of push lever 525. Therefore, the urging force of auxiliary spring 529 does not act as a force for rotating push lever 525.
On the other hand, as indicated by the two-dot chain line in FIG. 13, when the clutch lever is pulled, push lever 525 is pulled along and pivots counterclockwise. As a result, the attitude of auxiliary spring 529 changes, and the expansion/contraction direction M2 of auxiliary spring 529 shifts from pivot center C0 of push lever 525. A part of the urging force of auxiliary spring 529 thus acts as a force for pivoting push lever 525 counterclockwise. Therefore, the urging force of auxiliary spring 529 acts as an assist force applied when pulling the clutch lever, thereby reducing the operating force for operating the clutch lever.
In the above-described clutch operation assisting device, auxiliary spring 529 changes its attitude while the clutch lever moves from an initial position where the clutch lever is in the freeplay condition to a disengage position for disengaging the friction clutch. In the initial position, the expansion/contraction direction M1 of auxiliary spring 529 passes through the axial center (rotation center C0) of push screw 517. Accordingly, auxiliary spring 529 must be installed along the radial direction from the axial center of push screw 517, which makes it difficult to secure a sufficient installation space for auxiliary spring 529. Further, it is difficult to secure a sufficient expansion/contraction length for auxiliary spring 529. Therefore, the power unit must be enlarged to secure a sufficient installation space for auxiliary spring 529.